The present invention relates to a process for the production of ultra-pure oxygen by distillation of air, of the type in which a mixture containing oxygen, particularly air, is introduced into a distillation column which produces at its bottom a product rich in oxygen; an intermediate stream is withdrawn, substantially stripped of impurities heavier than oxygen, at a level of the column located above the point of introduction of said mixture, the point of withdrawal being spaced from this point of introduction of said mixture by a distillation section, particularly with packing; the withdrawn stream is sent to an oxygen purification column which produces at its bottom ultra-pure oxygen; and a liquid reflux is distributed over said section by means of a distributor comprising perforations for the passage of liquid and openings for the passage of gas.
In what follows, it will be supposed that the starting mixture is air.
In processes of this type, the presence of the distillation section located between the point of introduction of the air and the point of withdrawal of the intermediate stream, guarantees the substantially total absence in this flow, of impurities heavier than oxygen, and in particular methane, which is the lightest impurity among them. The elimination of these heavy impurities is carried out by the distillation section, to produce contents acceptable for the production of the desired ultra-pure oxygen.
The drawback of this method is that this number of plates required to eliminate the heavy products, also reduces the oxygen content at the level of withdrawal and thus renders distillation in the pure oxygen column more difficult, with the result of lower output of ultra-pure oxygen.
To limit this drawback, it has been proposed (U.S. Pat. No. 5,590,543) to reduce the quantity of reflux in the distillation section in question, by withdrawing from the distillation column a liquid fraction which supplies the section in question and by reinjecting it into the same column below the latter. As a result, the oxygen-nitrogen separation in the section is less effective, which leads to an increased oxygen content at the level of withdrawal.
The negative effect of this arrangement on the elimination of heavy impurities can be compensated by a slight increase of the number of theoretical plates (increasing the number of plates or of the height of the packing). This increase has little effect on the oxygen/nitrogen separation, because the latter takes place in the minimum reflux region, where the number of plates is a less sensitive parameter.
This solution is effective, but has a drawback: there is removed a liquid which is very pure as to heavy contaminants, and this liquid is reinjected directly into a fraction rich in heavy impurities (adjacent the air supply). This line of direct communication between the region in which it is desired to maintain a very high purity, and a region in which there exists a high concentration of pollutants, is undesirable, because the risk thus arises of accidental pollution of the oxygen purification column during stoppage or a transition phase of the installation or on the occasion of an erroneous manipulation.
The invention has for its object to obtain enrichment in oxygen of the intermediate flow with a higher degree of security.
To this end, the invention has for its object a process of the mentioned type, characterized in that there is carried out a predetermined deficiency in the uniformity of the distribution of the reflux liquid over the upper surface of said section in at least one localized region of this surface.
The process according to the invention can comprise one or several of the following characteristics:
the predetermined deficiency of uniformity is effected by withdrawing from the distillation column a controlled fraction of the liquid which supplies the distributor and by locally reinjecting into the same column this fraction below the distributor and over the upper surface of said section, such that the place or places on said surface which receive this fraction, receive a liquid flow substantially greater than that received by other positions on said surface;
the predetermined deficiency of uniformity is effected by creating a deficiency of uniformity of the perforations for passage of liquid from the distributor, in line with said region, relative to an arrangement of perforation leading to a distribution of liquid of optimum uniformity over said upper surface;
said localized region is adjacent the central axis of the column;
said localized region is adjacent the periphery of the distributor; and
all of the liquid which supplies the distributor and which is not sent to the purification column is sent onto said upper surface.
The invention also has for its object an installation for the production of ultra-pure oxygen by distillation of air, adapted to practice the process defined above. This installation, of the type comprising a distillation column provided with an introduction point for a mixture containing oxygen, particularly air, and a withdrawal point of an intermediate flow substantially free from impurities heavier than oxygen, the withdrawal point being located above the point of introduction of said mixture and being separated from the latter by a distillation section, particularly with packing; an oxygen purification column which produces at its bottom ultra-pure oxygen; a conduit which connects said point of withdrawal to the head of the oxygen purification column; and means for distributing reflux liquid over the upper surface of said section, these means comprising a distributor which comprises perforations for the passage of liquid and openings for the passage of gas, is characterized in that the distribution means are adapted to carry out a predetermined deficiency of uniformity of the distribution of the reflux liquid over the upper surface of said section in at least a localized region of said surface.
According to other characteristics of this installation:
distribution means comprising moreover a conduit for withdrawal from the distillation column, of a controlled fraction of the liquid which supplies the distributor, and for reinjection of this fraction in the same column, below the distributor, and over the upper surface of said section, such that the region or regions of said upper surface which receive said fraction receive a flow rate of liquid substantially greater than that received by other positions on said surface;
the distributor comprises a perforated bottom whose perforations have a predetermined non-uniformity relative to the arrangement of the perforations which would lead to an optimum uniformity of distribution of the liquid over said upper surface;
in line with said region, the distributor comprises perforations for the passage of supplemental liquid relative to an arrangement of perforations leading to a distribution of liquid of optimum uniformity over said upper surface;
in line with said region, the distributor comprises perforations for the passage of liquid, of increased or reduced dimensions, relative to an arrangement of the same perforations leading to a distribution of liquid of optimum uniformity over said upper surface; and
said localized region is adjacent the central axis of the column.